1. Field of the Invention
This invention relates to a safety device for a sewing machine, and more particularly to a safety interlock mechanism which prevents the sewing machine from malfunctioning.
2. Description of the Related Art
FIG. 9 shows a currently available and well-known sewing machine M which is capable of converting the stitch mode from "one needle two thread stitches A" (chain stitches) as shown in FIG. 7 to "one needle three thread stitches B" (overlock stitches) as shown in FIG. 8, and vice versa. The sewing machine M as shown includes needle N, upper and lower loopers (not shown) incorporated in a looper cover 6, a control unit for controlling the drives of these members, and a motor as a drive source.
A controller 13 in the form of a foot pedal is manipulated to drive the sewing machine M through the motor and to adjust the drive speed thereof. As is well known, the lower looper for the sewing machine is adapted to perform its sewing operation below a work table 5, whereas the upper looper is adapted to perform a predetermined sewing operation above the work table 5. A change-over dial or knob 1 selects the stitch mode as either "one needle two thread stitches A" or "one needle thread stitches B."
For formation of the "one needle two thread stitches A", the change-over dial or knob 1 is set to select a mark A on the machine cover, indicative of the "one needle two thread stitches A" and at the same time a second work table 3 as seen from FIG. 9 is installed in the sewing machine M. This will allow the sewing machine M to drive the needle N and the lower looper, rendering them ready for formation of the "one needle two thread stitches A" whereas the upper looper is lowered and inactivated in a position where the upper looper does not abut against the second work table 3.
When the "one needle three thread stitches B" mode is desired, the change-over dial or knob 1 is set to select the other mark B on the machine cover, indicative of the "one needle three thread stitches B," and at the same time a chip guard cover 4 (FIG. 1) is attached to the sewing machine M. This will allow the sewing machine M to drive the needle N and the upper and lower loopers, rendering them ready for formation of the "one needle three thread stitches B".
The aforementioned conventional sewing machine has disadvantages in that the upper looper is liable to collide with the second work table 3 if the change-over knob 1 is set to select the mark B indicative of the "one needle three thread stitches" with the second work table 3 attached to the sewing machine M as illustrated in FIG. 9. This may be dangerous to the operator since the second work table 3, the upper looper as well as the sewing machine M can break down and scatter fragments, thereby injuring the operator.